Conventional organic electroluminescent (OLED) devices are usually produced by deposition of the electrodes and the required thin organic electroluminescent layer(s) on a transparent substrate such as glass or a polymer foil through which the light is emitted. When a voltage between around 2 and 10 Volts is applied between the two electrodes the electroluminescent layer or stack of layers emits light. In such OLED devices the electrode deposited onto the substrate—usually referred to as substrate electrode and also usually forming the anode—can be deposited as thin layer of an electrically conducting but optically transparent oxide, typically indium-tin oxide (ITO). The electrode opposing the substrate electrode—usually referred to as counter electrode and also usually forming the cathode—is generally formed by evaporation of a layer of aluminum or silver with a thickness of around 100 nm after deposition of the electroluminescent layer(s).
The major advantage of OLED devices is the possibility of being able to produce thin light sources covering large areas. It is precisely in the case of large-area OLED layers covering a few square centimeters or more that the presence of particles, of dust for example, is unavoidable during the production process. Particles present on the substrate, such for example as dust particles of a diameter substantially greater than the thickness of the electroluminescent layer stack, cause defects, e.g. holes in the adjacent electroluminescent layer stack with edges of an undefined nature. No layered structure, or only a part of it, is present inside such hole. These defects result in unacceptable leakage currents and short-circuits between the substrate electrode and counter electrode. The short-circuits generally do not occur in this case until, in the course of operation of the OLED device, the operating voltage has to be increased, due to the decline in light yield, to allow the same amount of light to be generated. In contrast to a slow degradation of brightness due to the infiltration of oxygen or water into the light-emitting layers, failures of OLED devices as a result of short-circuits in the region of a defect become apparent as a sudden drop in brightness to zero. It is precisely in the case of large-area OLED devices that short-circuits in the region of hole defects are by far the most common cause of failing OLED devices.
Document WO2007/099476 discloses an electroluminescent arrangement, where an electrically isolating layer is deposited on top of the aluminum cathode as the counter electrode in order to react with the organic layers of the electroluminescent layer stack underneath the aluminum cathode, where the aluminum layer has a hole defect. The material of the isolating layer dissolves the organic layers underneath the aluminum cathode in a confined region around the hole defect. As a result, the distance between substrate electrode (anode) and aluminum cathode increases and thus the electrical field strength between anode and cathode decreases. A reduced electrical field strength around a defect in the electroluminescent layer stack lowers the risk of short-circuits caused by such hole defects. However, the remaining aluminum cathode layer around a hole defect still extend over the edges of the remaining organic layers around the hole defect and still has sharp edges of undefined nature. If the electrically isolating layer does not fill the cavity underneath the aluminum cathode completely and subsequently isolate cathode from anode perfectly, there is still a risk of short-circuits.